It is known in the prior art to use glasses having a chemically hardened surface, for example, as a scratch-resistant cover glass for displays in electrical devices. The chemical hardening of the glasses typically takes place by dipping a plate of the non-hardened glass into a salt bath. In this respect, an exchange of smaller ions (e.g. Na+) in the glass with larger ions (e.g. K+) from the salt bath takes place by diffusion in the regions of the plate close to the surface. The strength and the internal tension of the material in the diffusion layer is increased by the increased space requirements of the larger ions.
DE 10 2010 009 584 A1, for example, discloses a lithium-aluminosilicate glass at which a surface toughening first takes place by an exchange of lithium ions with sodium ions and in a second step by the exchange of sodium ions with potassium ions. A further chemically hardened glass having up to 20 mass % Al2O3 is disclosed in U.S. Pat. No. 8,075,999 B2.
It is the aim of the present disclosure to provide a glass which is suitable for a chemical hardening and which has improved properties.
This aim is achieved by an alkali-aluminosilicate glass which contains between 47.5 wt. % and 55 wt. % SiO2 and between 21 and 27.5 wt. % Al2O3. The glass furthermore contains between 12 and 16 wt. % sodium oxide Na2O. The molar ratio of Al2O3 to Na2O in the glass amounts to between 0.9:1 and 1.2:1.
Preferred ranges for the portion of SiO2 comprise between 50 and 55 wt. % and in particular between 53 and 55 wt. %. Preferred ranges for the portion of Al2O3 comprise between 21 and 25 wt. % and in particular between 21.5 and 23.5 wt. %. Preferred ranges for the portion of Na2O comprise between 13 and 16 wt. % and in particular between 13.8 and 15.6 wt. %.
In an embodiment, the molar ratio of Al2O3 to Na2O in the glass amounts to between 0.94:1 and 1.1:1.
In an embodiment, the molar ratio of SiO2 to Al2O3 in the glass amounts to between 2.5:1 and 4.8:1, preferably to between 3.5:1 and 4.5:1, and further preferably to between 3.9:1 and 4.2:1.
In an embodiment, the glass in accordance with the present disclosure comprises K2O, preferably between 1.5 and 2.5 wt. % K2O. In an embodiment, the glass in accordance with the present disclosure comprises Li2O, preferably up to 1.5 wt. % Li2O.
The glass in accordance with the present disclosure already has a greater hardness in the untreated state, i.e. in the not yet chemically hardened state, over other chemically hardenable alkali-aluminosilicate glasses. The glass in accordance with the present disclosure in the process of chemical toughening is furthermore characterized by a fast exchange speed and a considerable strength gain in the diffusion zone. There is no disadvantage in scratch-resistance with respect to known glasses having higher diffusion depths of the hardened layer due to the high base value of the hardness and the good properties during the chemical hardening, even with small dwell times in the salt bath with resulting thin diffusion layers. The glass already has a surface hardness before hardening, for example, of between 6400 MPa and 6600 MPa.
In an embodiment, the glass in accordance with the present disclosure is free of Ca or CaO. This can inter alia be advantageous with respect to the exchange rate of the sodium ions during chemical hardening.
In an embodiment, the glass in accordance with the present disclosure contains between 4 and 10 wt. %, preferably between 4 and 8 wt. %, and further preferably between 4.5 and 6 wt. % MgO. This can inter alia be advantageous with respect to the strength of the hardened surface layer. It is assumed that MgO has a network-strengthening effect in the region of the diffusion zone in the named concentration ranges.
In an embodiment, the glass in accordance with the present disclosure contains up to 1 wt. %, and preferably between 0.3 and 0.5 wt. % SnO2. In an embodiment, the glass in accordance with the present disclosure contains up to 0.6 and preferably up to 0.2 wt. % CeO2. In an embodiment, the glass in accordance with the present disclosure contains up to 1% F. These components can inter alia serve as fining means during the production of the glass.
In an embodiment, the glass in accordance with the present disclosure contains between 1.8 and 3.5 wt. %, and preferably between 2 and 2.5 wt. % ZrO2. In an embodiment, the aluminosilicate glass in accordance with the present disclosure contains between 0.2 and 3.5 wt. % ZnO.
The present disclosure further relates to a glass article comprising or consisting of an alkali-aluminosilicate glass in accordance with the present disclosure. The glass article can, for example, be produced solely from the glass in accordance with the present disclosure or can have zones, for example layers, of the glass in accordance with the present disclosure.
In an embodiment, the alkali-aluminosilicate glass of the glass article is chemically hardened at least sectionally at its surface and has a diffusion layer at least sectionally at its surface. In the diffusion layer, the concentration of Na is lowered with respect to the untreated glass composition and the concentration of another element, for example K, is increased with respect to the untreated glass composition. The diffusion layer can be achieved by a chemical hardening process, with the glass article being dipped into a salt bath, for example a bath of molten KNO3, at specific temperatures and dwell times.
In an embodiment, the diffusion layer is between 10 μm and 60 μm thick, preferably between 15 μm and 35 μm thick, and further preferably between 20 μm and 30 μm thick. The layer thickness depends on the dwell time or on the temperature in the salt bath. The border between the diffusion layer and the remaining glass is to be drawn where there is a measurable difference in the concentration of Na with respect to the untreated glass composition.
In an embodiment, the glass article is a glass pane which is either manufactured homogeneously from the glass in accordance with the present disclosure or has at least one layer of the glass in accordance with the present disclosure. In this case, one or both oppositely disposed surfaces of the pane can be chemically hardened. The pane or layer of the glass in accordance with the present disclosure can be between 0.4 mm and 2 mm thick.
The present disclosure further relates to the use of such a glass pane as a component of electronic devices, in particular as a cover glass for displays, optionally with a touch screen. Suitable electronic devices in particular comprise cell phones or smart phones and computers or tablet computers.
Further details and advantages result from the following FIGURE and from the embodiments.